1. Field of the Invention
The present invention relates to an image display apparatus (projector), more particularly, although not exclusively, an image display apparatus which can use reflective liquid crystal elements.
2. Description of the Related Art
Since the reflective liquid crystal panel has advantages over a transmissive liquid crystal panel (e.g., high numerical aperture, high definition) attention is being given to a projector which can use reflective liquid crystal panels. Such a projector can switch over between display (a bright state to project light) and non-display (a dark state not to project light) of each pixel of an image on a screen through modulation of each reflective liquid crystal panel. Further, corresponding polarizing beam splitters are arranged in front of multiple liquid crystal panels, respectively, in the projector to separate light from a light source into multiple color beams and combine the color beams from the liquid crystal panels. Therefore, a color separating/combining system of such a reflective liquid crystal projector generally tends to be large in size.
In order to address this problem, US patent publication 2002/0140905 A1 discusses a projector which can use three polarizing beam splitters to perform color separation and combination in cooperation with three reflective liquid crystal panels for three primary colors. FIG. 7 illustrates the structure of the projector.
A polarization converting element 3a located in the course of an illumination optical system brings these beams to the same s-polarization state. As a result, these beams turn into an s-polarized red beam 4R, an s-polarized green beam 4G, and an s-polarized blue beam 4B, respectively. The optical element 5 is dichroic mirror 5 has the property of reflecting primarily green light. The color beams 4R and 4B that passed through the optical element 5 pass through a polarizing plate 7a to increase their degree of polarization, and enter a wavelength-selective polarization rotating element 9. The wavelength-selective polarization rotating element 9 has such a property as to rotate the polarization direction of the blue light component 90 degrees while not rotating the red light component. After passing through the wavelength-selective polarization rotating element 9, the red beam and the blue beam that are originally s-polarized light turn into an s-polarized red beam 10R and a p-polarized blue beam 10B, respectively, and enter a polarizing beam splitter 11. The beam 10R incident on the polarizing beam splitter 11 is reflected on a polarization split face 11a to enter a reflective liquid crystal panel 13R. When a corresponding pixel in the reflective liquid crystal panel 13R is in an ON state (such a bright state to project light on a screen) and the red beam 10R enters the pixel, it turns into a p-polarized red beam 14R and enters the polarizing beam splitter 11 again. The p-polarized red beam 14R passes through the polarization split face 11a and exits from the polarizing beam splitter 11. On the other hand, the blue beam 10B is p-polarized, so that it passes through the polarization split face 11a and enters a reflective liquid crystal panel 13B. When entering an on-state pixel in the reflective liquid crystal panel 13B, the blue beam 10B turns into an s-polarized blue beam 14B, and enters the polarizing beam splitter 11 again. Since the blue beam is s-polarized, it is reflected on the polarization split face 11a and exits from the polarizing beam splitter 11.
On the other hand, the beam 4G reflected by the optical element 5 passes through a polarizing plate 25 to increase its degree of polarization, and enters a polarizing beam splitter 27 to reach a polarization split face 27a. Since the green beam 4G is s-polarized, it is reflected on the polarization split face 27a and enters a reflective liquid crystal panel 29G. When entering an on-state pixel in the reflective liquid crystal panel 29G, the green beam 4G is converted (modulated) to a p-polarized green beam 30G and enters the polarizing beam splitter 27 again. The p-polarized green beam 30G is transmitted through the polarization split face 27a and exits from the polarizing beam splitter 27.
Also illustrated is a wavelength-selective polarization rotating element 16. The wavelength-selective polarization rotating element 16 has such a property as to rotate the polarization direction of the blue light component 90 degrees while not rotating the red light component. After passing through the wavelength-selective polarization rotating element 16, the red beam and the blue beam turn into a p-polarized red beam 19R and a p-polarized blue beam 19B, respectively, and enter a polarizing beam splitter 20. Also illustrated is a half-wave plate 34. The half-wave plate 34 has such a property as to rotate the polarization direction 90 degrees. After passing through the half-wave plate 34, the green beam 31G is an s-polarized green beam 35G, and enters the polarizing beam splitter 20. Then the beam 35G is reflected by the polarization split face 20a to reach a projection lens 24. Then the beams 19R,19B pass the polarization split face 20a and reach a projection lens 24.
This patent publication discusses the use of a polarizing beam splitter 11, two reflective liquid crystal panels 13R and 13B, wavelength-selective polarization rotating elements 9 and 16 arranged on the light incident and exit sides of the polarizing beam splitter 11 to perform display/non-display selection (control) of color beams from two reflective liquid crystal panels 13R and 13B.